Theiler's murine encephalomyelitis virus (TMEV)-induced de-myelinating disease (IDD) is believed to be one of the best viral models of multiple sclerosis (MS). TMEV-IDD has been shown to be genetically regulated by genes both in and outside the major histocompatibility complex (MHC). The best immunological parameter that correlates with susceptibility of a particular strain to this demyelinating disease is its ability to develop a delayed-type hyper-sensitivity (DTH) response to viral antigens. Both immunological parameters, such as the ability to develop a DTH response to virus, as well as factors regulated at target cell level may be responsible for differential susceptibility to TMEV-IDD in different murine strains. DTH responses are driven by CD4+ Tho cells which are negatively regulated by CD4+ The cells. These two different populations of helper lymphocytes secrete different cytokines that are important in the development of the inflammatory response. The main hypothesis of this proposal is that strains with different susceptibility TMEV-IDD may show different expression of these important cytokines in parallel with their susceptibility or resistance to TMEV-IDD. Since chronic DTH responses depend on the persistence of the eliciting antigen, Dr Dal Canto proposes also to study the ability of various CNS cells from murine strains with different susceptibility/resistance to TMEV-IDD to become infected by TMEV, and will explore the ability by these cells to express a number of important soluble and surface molecules which are believed to be important in the development and progress of the inflammatory response in the CNS. This proposal is articulated into four Specific Aims. Studies will be both in vivo and in vitro.. In a 1, the principal investigator proposes to explore the possible differential susceptibility to infection by astrocytes and oligodendrocytes from strains with different susceptibility to TMEV-IDD. Both survival of the cells and their most important luxury functions will be studied. In Aim 2, the principal investigator proposes to investigate the role of different sub-sets of inflammatory cells in lesion formation in a temporal fashion, and to focus particularly on the balance between Th1 and Th2 cells by studying their specific lymphokines. In addition, the role of astrocytes as immune-competent cells will be investigated. Aim 3 will be to manipulate the expression of IL-4, the most important cytokine for the differentiation of Th2 lymphocytes, to see whether the development and/or severity of TMEV-IDD can be controlled. Aim 4 will test the DTH hypothesis more directly by mutating the major DTH epitope of the virus. It is expected that disease will be either prevented or significantly reduced in its severity in mice infected with the mutated virus.